The present invention relates to a liquid metal thermal electric converter (LMTEC). The LMTEC is a thermodynamic heat engine which converts heat directly to electricity. More particularly, the present invention relates to a LMTEC having a unique .beta."-alumina tube configuration. Further, the present invention relates to a multiple-cell LMTEC having a refluxing boiler with a series connection of cells.
A device which converts heat energy directly to electric energy generally comprises an enclosed container which is separated into a first and second reaction zone by means of a solid electrolyte. A liquid metal is present in the first reaction zone (i.e., on one side of the solid electrolyte) and during operation of the device, the first reaction zone is maintained at a temperature T.sub.2 and a pressure P.sub.2 which is higher than the temperature T.sub.1 and pressure P.sub.1 of the second reaction zone, which creates a metal-vapor pressure differential between the two reaction zones. In the lower pressure second reaction zone, a porous or permeable electrically conducting electrode is in contact with the solid electrolyte. Further, during operation a heat source maintains the temperature of the liquid metal within the first reaction zone at the high temperature T.sub.2 and corresponding high vapor pressure P.sub.2. Metal ions are caused to migrate through the solid electrolyte by the pressure differential while the electrons are caused to move through an external circuit to do useful work and then come in contact with the porous electrode, wherein the metal ions are neutralized to their elemental state at the solid electrolyte-porous electrode interface. The elemental metal is then caused to evaporate from the porous electrode and to migrate through the low pressure P.sub.1 second reaction zone (i.e., a vacuum space) to a low temperature T.sub.1 condenser and form a condensed liquid metal. The condensed liquid metal is then returned to the higher temperature region within the first reaction zone, e.g., by means of a return line and a pump, to complete a closed cycle. Thus, in summary, during operation of the device, the metal passes from the first reaction zone to the second reaction zone and is pumped back to the first reaction zone. The process occurring in the solid electrolyte and at the solid electrolyte-porous electrolyte interface is approximately equivalent to an isothermal expansion of the metal from pressure P.sub.2 to P.sub.1 at the temperature T.sub.2.
The process of direct energy conversion is characterized by the independence of size vs. efficiency, the absence of moving parts, high reliability, quietness, lack of vibration, low maintenance, simple startup, and absence of pollution problems. Further, the work output of the process is electrical only.
Exemplary thermoelectric devices to which the improvement of the present invention applies and the principles of operation thereof have been described in U.S. Pat. Nos. 3,458,356, 4,098,958, 4,220,692, 4,505,991 and 4,510,210. "Sodium Heat Engines" (SHE) is the name commonly given to such thermoelectric devices which electrochemically expand sodium metal across a solid electrolyte. While other metals may be employed in the device of this invention, the sodium heat engine is described herein as exemplary of such devices. The SHE design places the porous electrode on the outside of the .beta.-alumina tube and the high temperature (high pressure) liquid sodium on the inside of the tube. This configuration creates tensile stresses in the ceramic .beta.-alumina tube. The choice of sodium as the working fluid minimizes these stresses to a low level, but requires the engine to operate at a relatively high temperature, 700.degree. to 1,000.degree. C., for modest efficiencies.
U.S. Pat. No. 4,042,757 discloses a system where high-pressure sodium is on the outside of an alumina tube and a porous electrode is on the inside. Because the condensor of this patent is inside the tube, it has higher radiative losses than would a remote condensor. In addition, the use of the porous electrode for the conduction of current along the length of the tube could limit current flow through the relatively high resistance electrode.
Since the power from a SHE or LMTEC is limited by the area of the .beta."-alumina tube, for a given tube size, multiple tubes must be used to increase the power output. The SHE or the LMTEC is also a low voltage, high current device. Therefore, when multiple tubes are used it is very desirable to connect them in series. The SHE design has liquid sodium completely filling the .beta."-alumina tube, thereby limiting the ways that the .beta."-alumina tubes can be connected in series. For example, the tubes could be isolated, with a sodium reservoir for each tube. This option is basically the series connection of single tube SHE's, and it requires multiple pumps, boilers and condensors. Another option is to supply the series connected tubes from a single sodium reservoir through small sodium lines. This configuration results in internal electrical shorting of the series connected tubes through the sodium supply lines, and it limits the number of series connected tubes.